A medical device (MD) can be implanted in a patient for various different purposes, including, but not limited to, treating physiologic conditions, monitoring physiological conditions, treating neurological conditions, monitoring neurological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Where the MD is implanted, it is often referred to as an implantable medical device (IMD). Examples of IMDs include, but are not limited to, implantable neurostimulators, implantable cardiac rhythm management devices (e.g., implantable cardioverter defibrillators and pacemakers) and drug delivery devices. Because such a device may be implanted in a patient, the size of the device is inherently constrained. For this and other reasons, an IMD may depend on an external (i.e., non-implanted) system, generically referred to as a base station (BS), to perform certain functions. Such a non-implanted BS can be a patient programmer, a clinician programmer or a remote monitoring device, but is not limited thereto.
An implantable neurostimulator (INS) is an IMD that performs neurostimulation, which has become an accepted treatment for patients with chronic pain in their back and/or limbs who have not found pain relief from other treatments. In general, neurostimulation involves applying an electrical current to nerve tissue in the pathway of the chronic pain. This creates a sensation that blocks the brain's ability to sense the previously perceived pain. There are two conventional forms of electrical stimulation commonly used to treat chronic pain: Spinal Cord Stimulation (SCS) and Peripheral Nerve Field Stimulation (PNFS). In SCS, electrical leads are placed along the spinal cord. A programmable INS is typically implanted in the upper buttock or abdomen (under the skin) and emits electrical currents to the spinal cord via electrodes of the leads. Peripheral nerve field stimulation is similar to spinal cord stimulation, however peripheral nerve field stimulation involves placing the leads just under the skin in an area near to the peripheral nerves involved in pain.
Leads are often attached to an IMD, such as an INS, to deliver electrical stimulation via electrodes of the leads. An IMD often includes a hermetically sealed device housing within which is located electronic circuitry used for generating and controlling the electrical stimulation, and a header which is used to connect the leads to the IMD. The header is often molded from a relatively hard, dielectric, non-conductive polymer and typically has a thickness approximating the thickness of the device housing. The header typically includes a mounting surface that conforms to and is mechanically affixed to a mating sidewall surface of the device housing.
Wireless communication between an IMD and an external BS is often referred to as telemetry. Examples of specific telemetry functions include, but are not limited to, programming or instructing the IMD to perform certain therapeutic tasks and/or adjust certain therapeutic parameters, downloading firmware upgrades to the IMD, uploading operational status information (e.g., battery and/or impedance measurements) from the IMD, and uploading data stored within the IMD. A useful type of wireless communication is radio frequency (RF) communication since it does not require that the BS and the IMD be very close to one another. Rather, with RF communication the BS and the IMD can be many feet apart while still allowing for reliable communication.
A non-implanted BS and an IMD, such as an INS, can communicate using the Medical Implant Communication Service (MICS) standard, which was defined by the U.S. Federal Communications Commission (FCC) and European Telecommunications Standards Institute (ETSI). The MICS standard uses the RF band between 402 and 405 MHz to provide for bi-directional radio communication with IMDs, such as an INS. The RF band between 402 and 405 MHz can be broken down into multiple channels, e.g., into ten 300 kHz wide channels, but not limited thereto. In 2009 the FCC began referring to the RF band between 402 and 405 MHz as being part of the 401 to 406 MHz Medical Device Radiocommunications (MedRadio) Service band. Accordingly, for the remainder of this description, the RF band between 402 and 405 MHz will be referred to as the MICS/MedRadio band, and the communication standards relating to the MICS/MedRadio band will be referred to as the MICS/MedRadio communication standards. The use of other frequencies, e.g., in the range from 300 MHz through 1 GHz, but not limited thereto, are also possible. Further possible frequencies that can be used include industrial, scientific and medical (ISM) radio bands, such as, but not limited to, the 2.45 GHz and the 5.8 GHz bands, as well as much lower frequency bands.
An IMD, such as an INS, includes an antenna for use in receiving signals from a BS and transmitting signals to the BS. The antenna can be, for example, located within the hermetic device housing of the IMD, or within the header of the IMD. A benefit of locating the antenna within the header (as opposed to within the hermetic device housing) is that the antenna is generally isolated from electronic circuitry of the IMD, and thus, is generally not inadvertently affected by the electronic circuitry. Another reason to not locate an antenna within the hermetic device housing is that the sealed metal housing can prevent the antenna from radiating, i.e., the metal housing can shield the antenna. However, a challenge with locating the antenna within the header is that the header is small, and a relatively large portion of the header is already devoted to providing mechanical and electrical connections to the proximal ends of one or more therapy leads.
FIG. 1 illustrates an exemplary IMD 112 that includes a hermetically sealed device housing 110, which is typically made of medical grade metal. Contained within the housing 110 is electronic circuitry 118 used for generating and controlling the electrical stimulation, and a header 120 which is used to interconnect leads 114 to the IMD 112. The electronic circuitry 118 is shown as including therapy circuitry 108 and telemetry circuitry 106. The housing 110 is also shown as containing a battery 104 that is used to power the electronic circuitry 118.
A header 120, which is typically made of a medical grade polymer or other plastic, is mechanically affixed to a mating surface 111 of the device housing 110. As shown in FIG. 1, a portion 122 of the header 120 includes connectors (e.g., bores or sockets) that accept proximal ends of the leads 114 to thereby mechanically connect the leads to the header 120. Electrical conductors, e.g., wires and/or conductive traces, extend from the header 120 through feed-through openings 113 in the mating surface 111 of the device housing 110 to thereby electrically connect the leads 114, and the electrodes thereon, to the therapy circuitry 108.
As disclosed in U.S. Pat. No. 6,708,065 to Von Arx et al. (the '065 patent), a helical antenna can be embedded in the header. The 065' patent explains that two common types of antennas are wire dipole and monopole antennas. If a substantial portion of the RF energy delivered to the antenna is to be emitted as far-field radiation, the length of the antenna should not be very much shorter than one-quarter of the wavelength of the RF carrier signal provided by the RF transmitter. For implantable medical device applications, carrier frequencies between 300 MHz and 1 GHz are most desirable. For example, the carrier signal can be 1 GHz, which corresponds to a wavelength of approximately 30 cm. For a 30 cm wavelength, a half-wavelength dipole antenna would optimally be approximately 15 cm (i.e., 150 mm) long, and a quarter-wavelength monopole antenna would optimally have a length of approximately 7.5 cm (i.e., 75 mm) with the housing serving as a ground plane. Depending upon the size of the implantable device, it may not be possible or convenient to embed a straight wire antenna in a compartment of the device. For reasons of patient comfort, however, it is desirable for an implanted device to be as small as possible, and this constrains the length of the antenna that can be used if it is to be embedded in a compartment of the device.
The '065 patent explains that it employs a helical antenna to transmit and receive RF signals. The '065 patent also explains that its helical antenna is formed by helically coiling a length of wire or other conductor along a particular axis. If the circumference of the individual helices is small in comparison to the wavelength of the driving or received signal, the radiation pattern of the helical antenna is approximately the same as either a dipole antenna or a monopole antenna if a ground plane is present. A helical dipole or monopole antenna may be formed by coiling a length of wire corresponding to just over one-half wavelength or one-quarter wavelength of the carrier frequency. Owing to the coiling of the wire, the resulting helical antenna is physically shorter than the monopole or dipole antenna formed from the straight piece of wire. The effective electrical length of a helical antenna, however, is even longer than that owing to the added inductance of the coil and turn-to-turn capacitance which reduces the resonance frequency from that of the corresponding straight wire antenna. A helical antenna thus provides a shortened, space-saving monopole or dipole antenna that behaves electrically like a much longer antenna.
FIGS. 2A, 2B and 2C illustrate, respectively, how the '065 patent (in FIGS. 1A, 1B and 1C of the '065 patent) teaches locating and positioning a helical antenna 200 within the header 120. In FIG. 2A (which is similar to FIG. 1A of the '065 patent), the helical antenna 200 is positioned roughly parallel to the mating surface 111 of the device housing 110. In FIG. 2B (which is similar to FIG. 1B of the '065 patent), the helical antenna 200 is positioned perpendicular to the surface of the device housing 110. In FIG. 2C (which is similar to FIG. 1C of the '065 patent), the helical antenna 200 is helically wound around one of the bores into which a proximal end of a therapy lead inserts.
A helical antenna (e.g., 200), such as the antenna disclosed in the '065 patent, is an antenna having the shape of a helix. A helix is a smooth curve in three-dimensional space characterized by the fact that the tangent line at any point makes a constant angle with a fixed line called the axis. Another way of explaining a helix is the curve formed by a straight line drawn on a plane when that plane is wrapped around a right circular cylinder.
The '065 patent does not discuss whether or how the diameter or radius of a helical antenna affects performance of the antenna. Rather, the '065 patent only suggests how to select a length of a helical antenna. Further, from FIGS. 1A-1C of the '065 patent, the diameter and radius of the helical antenna shown therein appear to be small compared to the relative space available for locating the antenna within the header of the exemplary implantable device shown therein.
U.S. Pat. No. 6,505,072 to Linder et al. (the '072 patent), in FIG. 3 of the '072 patent, also discloses that a helical antenna can be disposed in the header of an IMD. However, the '072 patent also does not discuss whether or how the diameter or radius of a helical antenna affects performance of the antenna. Further, from FIG. 3 of the '072 patent, the diameter and radius of the helical antenna shown therein appears to be small compared to the relative space available for locating the antenna within the header of the exemplary implantable device shown therein.